"Things were humming along nicely, but then one of the components of the transplant started failing," he says. "We kind of waited it out a couple of weeks to see if it would fail entirely."

His situation did not improve, which led to the donor infusion on Tuesday.

"This will hopefully prevent it from failing," he says. "If they didn't do this, it would certainly have failed."

With doctors saying there is only a 50/50 chance that the infusion will work, Madoff insists he is trying to stay positive.

"It's hard. It's been a long road. I've been receiving treatment since January, and it's a lot," he says, ticking off, "Six rounds of chemotherapy, then radiation, then more chemo before the transplant, then the transplant itself."

Madoff, whose stage one cancer (confined to a single lymph node), was originally diagnosed in 2003, now has stage four cancer, which has spread throughout his body. He revealed his recurrence of the disease to PEOPLE earlier this year, after his condition had been diagnosed during his annual checkup last October.

He says that after the PEOPLE story ran he received hundreds of emails and letters of support from total strangers.

"No matter what ends up happening, those well wishes weren't for naught," he says. "They've made a huge difference to me in making me feel good and stay positive."

In April, Madoff moved to Seattle to undergo the transplant at the Fred Hutchinson Cancer Research Center. If this week's procedure doesn't work, he says doctors can try it again in a few weeks.

As for his father, who is serving his 150-year sentence at a federal prison in North Carolina, Andrew says he has not heard from him since he revealed his cancer has returned. Regardless, his feelings toward his father have not changed.

"Even on my deathbed I will never forgive him for what he did," he says.

"I just wanted to express my deep gratitude for all the love and support," he adds. "It's meant so much to me. It's been so helpful. I need it now more than ever."